Short answer
On multi-unit builds I seal unit by unit, measuring each one live and stopping when it reaches its target, so every unit leaves with its own documented result. Small units are the hardest to pass because the ACH50 math divides leakage by a small volume, which is where an aerosol seal earns its keep. Firestopping and fire separations are still done by hand to code; the seal works alongside them, not instead of them.
Why small units fail more often
ACH50 is the leakage at 50 pascals divided by the volume of the space. A compact suite has a small volume, so the same leakage that would pass in a large house produces a failing number in a small one. Add shared walls, more penetrations per square foot, and more trades in a tighter space, and multi-unit builds miss more often than single-family homes do.
Each unit also has to be considered on its own. One leaky unit can hold up occupancy on a building where the others are fine.
How I seal a multi-unit building
- Unit by unit. Each unit is closed up, pressurized and sealed with its own baseline and closing reading.
- Measured per unit. Every unit gets its own Airtightness Verification Report, so your energy advisor and your file have a clean record per door.
- Sequenced with your schedule. Units can be sealed as they reach the right stage rather than waiting for the whole building.
- Selective sealing. If only some units need it, I seal only those.
Where units share walls, the way the test is set up matters. Measuring one unit while its neighbours are at a different pressure reads leakage between units as leakage to outside. When a project needs it, the neighbouring units are pressurized at the same time (a guarded test) so the number reflects the exterior envelope. Your energy advisor decides how the compliance test is run; I set up the seal to match.
On a recent Kamloops fourplex, I sealed all four units in one day against a Step 4 target, and every unit finished between 0.48 and 0.74 ACH50 on my gauge before the energy advisor tested.
Compartmentalization: the side benefit owners notice
Sealing the leakage paths between units also cuts the air that moves between them: cooking smells, smoke and noise that travel through gaps at shared walls and floors. That is a selling point for a strata or a rental, not just a code number.
The seal is not a fire-rated product and does not replace firestopping or rated fire separations. Those are designed and installed to code first. The seal closes the remaining small leakage paths.
Rental financing and FortisBC programs
If your project is financed through CMHC MLI Select, the energy points come from your energy model, not from a blower door number. CMHC does not set an airtightness requirement. What changed is the baseline: for new construction, CMHC now measures energy savings against the 2020 national codes. The top energy tier needs at least 70% better than 2020 NBC Tier 1 on Part 9, or at least 60% better than 2020 NECB Tier 1 on Part 3. The borrower signs an attestation that the energy criteria were met within 60 days of the final advance.
Airtightness is one of the inputs in that model that can come in worse than assumed, especially in compact suites with shared walls. A measured seal gives your energy advisor a real number to work with instead of a hopeful one.
- FortisBC Commercial New Construction Program. For Part 3 buildings and some low-rise Part 9 multi-unit buildings modelled with the Part 3 method. Multi-unit residential buildings target Step 4. It includes a rebate on the mid-construction airtightness test. You enrol in the design phase, before construction starts.
- FortisBC New Home Program. For duplexes, triplexes, townhouses and other attached homes up to three storeys, at Step 4 or Step 5. It pays only on completed homes with an occupancy permit, and it is scheduled to end February 28, 2027, or sooner if the funding runs out.
Program details change. Check the current terms with CMHC, your lender, your energy advisor and FortisBC before you rely on them. I am an air sealing contractor, not an energy advisor; your energy advisor runs the model and the compliance test.
Larger buildings and Part 3 rental
Most of my multi-family work is Part 9: duplexes, fourplexes, townhouses and small apartment buildings. I also take on Part 3 rental buildings. On Part 3, the BC Building Code does not set an airtightness level, but it does require the building to be tested at 75 pascals or more, and the energy model has to use the air leakage rate measured on the building once that test is done. If the building tests leakier than the model assumed, the margin you designed to shrinks.
Send me the drawings, the unit count and the air leakage rate your modeller assumed. I will tell you whether a seal is practical, which floors or units it makes sense on, and how it would be phased with your schedule.
Written by Sebastian Motora, owner-operator. Last reviewed October 7, 2026.